1. Field of the Invention
The present invention relates to a spatial light modulator, and a display device including the spatial light modulator.
2. Description of the Related Art
In the related art, for example, Japanese Laid-Open Patent Application No. 11-258585 (referred to as “reference 1” hereinafter) discloses a display device including a reflection type spatial light modulator, such as a reflection type liquid crystal displaying element. This reflection spatial light modulator has a lens-array formed on a light transmission substrate. This lens-array prevents incidence of light on an area between two pixel electrodes where liquid crystal molecules are not sufficiently aligned, thereby obtaining an image of high brightness and high quality.
Japanese Patent Gazette No. 3239969 (referred to as “reference 2” hereinafter) discloses a display device including a transmission type spatial light modulator, such as a transmission type liquid crystal displaying element, which includes a micro-lens array to optically reduce the size of a pixel of the displaying element. In addition, by using a pixel-shift function of a wobbling element or others, positions of pixels are shifted by a specified distance on a screen. Because the pixel size is reduced by the micro-lens array, and the pixels are further shifted by using the pixel-shift technique, overlapping of adjacent pixels is reduced, and high-resolution images can be obtained.
Japanese Laid-Open Patent Application No. 8-186780 discloses a display device which uses a micro-lens array and a wobbling element, and the pixel size of the liquid crystal displaying element is reduced by using the micro-lens array.
As described so far, it is a well-known technique to provide a micro-lens array in a spatial light modulator (for example, a liquid crystal displaying element) to condense illumination light to a position near the center of a pixel, and thus, the illumination light otherwise going to a region between pixels (that is, a non-pixel region) can be deflected to a pixel. Thereby the efficiency of light utilization can be improved, and degradation of a contrast ratio can be prevented, which can be attributed to a fringe effect caused by non-uniformity of the electric field surrounding each pixel.
In addition, also as described above, it is a well-known technique to provide a pixel-shift element in a display device. By providing the pixel-shift element in a light path of projecting light so as to rapidly shift the light path, for example, the pixel-shift element shifts the light path by half a pixel in synchronization with an image updating operation in the spatial light modulator, it is possible to apparently display a larger number of pixels than the actual number of pixels of the spatial light modulator, for example, a number of pixels equaling an integral multiple of the pixel number of the spatial light modulator are displayed.
Developments are being made of such kinds of spatial light modulators and display devices including the micro-lens array to further reduce overlapping of the projected pixels and improve the image resolution.
In the spatial light modulator having a micro-lens, it is required to maintain not only high geometric optical properties of the micro-lens, but also a high contrast ratio. In a projector including the spatial light modulator, the illumination light is incident on the spatial light modulator, and is modulated by the spatial light modulator according to image data. The modulated illumination light which carries image data is projected to form images on a screen.
In order to obtain uniform images, the illumination light should be uniformly emitted on the spatial light modulator, and for this purpose, light integrators such as fly's-eye lenses or rod lenses are employed.
In illumination optical systems used for these purposes, the illumination light is incident on the spatial light modulator at various incident angles defined by an F value of the optical system, and especially for the light having a large incident angle, the plane of polarization changes due to the micro-lens.
In addition, in display devices as described above, in order to further improve the contrast ratio, wave plates are provided in the light path so as to correct the residual phase difference caused by the spatial light modulator, or correct skewness of light caused by a polarized light splitting element, such as a polarization beam splitter.
Inventors of the present invention found the following problems in the above illumination optical systems. In the above illumination optical system using the wave plate for compensation for the residual phase difference, the contrast ratio decreases when the curvature radius of the micro-lens is small and the lens power is large. This is caused by rotation of the polarized light and especially by a difference of the compensation condition for the light of a large incident angle (angle of incidence). Further, in a reflection spatial light modulator, the angle of the incident light entering the wave plate may differ from the angle of the returning light which leaves the wave plate, and this causes further decrease of the contrast ratio.
In addition to the above problems, in the aforesaid display device, which uses the micro-lens array (MLA) to reduce the apparent pixel size of the spatial light modulation element and uses the pixel-shift function of the wobbling element to shift positions of pixels on a screen, after the illumination light is refracted by the micro-lens array, the incidence plane of a portion of the illumination light becomes not parallel or perpendicular to the polarization plane of the polarized light. In this case, even when the illumination light is linearly polarized, the illumination light ends up having a P polarization component and an S polarization component in the refraction plane. Because the P polarization component and the S polarization component of the illumination light have different transmittances, the polarization plane rotates after refraction by the micro-lens. For this reason, the polarization state of the illumination light after the micro-lens array changes. When such kind of illumination light is incident on the spatial light modulator, even in a dark state (no modulated light arrives at the projecting lens), leakage of the modulated light occurs, and the leaked modulated light arrives at the projecting lens and is projected onto the screen, causing decrease of the contrast ratio of the displayed image.
In order to solve the above problem, that is, when the micro-lens is used in the spatial light modulator to increase the efficiency of light utilization, and the contrast ratio decreases due to rotation of the polarization plane caused by refraction of the lens, the inventors of the present invention proposed a display device including a spatial light modulator (for example, a liquid crystal display panel) with an anti-reflection element being provided on a refracting side of the micro-lens, that is, a side possessing refraction power, and proposed a display device including this spatial light modulator and a pixel-shift element.
In this display device, the spatial light modulator has a plural number of pixels, and includes spatial light modulation elements each for modulating the polarization state or intensity of the light incident on one pixel. In the spatial light modulator, a micro-lens array is arranged near the spatial light modulation elements, and the micro-lens array has a lens pitch equaling the pixel pitch of the spatial light modulation elements. In addition, an anti-reflection element is provided on a refracting side of the micro-lens array so as to prevent the decrease of the contrast ratio. By providing the anti-reflection element, the polarization plane of the illumination light rotates very little when the illumination light is refracted by the micro-lens array, and this prevents the decrease of the contrast ratio.
A λ/4 wave plate is often used in a display device. The above display devices, however, are not proposed to include a configuration with a λ/4 wave plate being placed between the micro-lens array and the liquid crystal layer. In addition, when a wave plate is arranged between a polarized light splitting element and a micro-lens array, when the illumination light propagates in the wave plate, the angle of the illumination light is different from the angle of the projected light (that is, the light projected to the screen to form an image), and the spread of the projected light is larger than that of the illumination light. For this reason, phase of the illumination light differs from the phase of the projected light, and due to this, the screen in the dark state is somehow bright, that is, causing a decrease of the contrast ratio.